<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .one{
            width: 200px;
            height: 100px;
            border: 3px solid black;
            margin: 5px;
            float: left;
        }
        .big{
            border: 3px solid red;
        }
        .clear{
            clear: both;
        }
    </style>
</head>
<body>
    <div class="big">
        <div class="one">one</div>
        <div class="one">one</div>
        <div class="one">one</div>
        <div class="clear"></div>
    </div>

</body>
</html>